This invention relates to thermally tunable fiber devices and, in particular, to thermally tunable devices disposed within microcapillary heaters.
Optical fibers include within their structures a variety of devices highly important for the proper operation of systems employing the fibers. Such devices, designed to process entering light include optical gratings and lengths of specialized fiber such as dispersion compensating fiber and rare earth doped amplifying fiber. In many applications it is desirable to tune selected characteristics of the fiber devices. Tuning by the application of heat is particularly convenient, especially for fiber grating devices.
Optical fiber gratings are important elements for selectively controlling specific wavelengths of light within optical systems. An optical grating typically comprises a body of material and a plurality of substantially equally spaced optical grating elements such as index perturbations, slits or grooves. Such gratings include Bragg gratings and long period gratings. The ability to dynamically modify these gratings would be highly useful.
A typical Bragg grating comprises a length of optical waveguide, such as optical fiber, including a plurality of perturbations in the index of refraction. These perturbations selectively reflect light of wavelength xcex equal to twice the spacing xcex9 between successive perturbations times the effective refractive index, i.e. xcex=2neffxcex9, where xcex is the vacuum wavelength and neff is the effective reactive index of the propagating mode. The remaining wavelengths pass essentially unimpeded. Bragg gratings have found use in a variety of applications including filtering, adding and dropping signal channels, stabilization of semiconductor lasers, reflection of fiber amplifier pump energy, and compensation for waveguide dispersion.
A long period grating couples optical power between two copropagating modes with very low back reflections. It typically comprises a length of optical waveguide wherein the refractive index perturbations are spaced by a periodic distance xcex9xe2x80x2 which is large compared to the wavelength xcex of the transmitted light. In contrast with conventional Bragg gratings, long-period gratings use a periodic spacing xcex9xe2x80x2 which is typically at least 10 times larger than the transmitted wavelength, i.e. xcex9xe2x80x2xe2x89xa710xcex. Typically xcex9xe2x80x2 is in the range 15-1500 micrometers, and the width of a perturbation is in the range 1/5 xcex9xe2x80x2 to 4/5 xcex9xe2x80x2. In some applications, such as chirped gratings, the spacing xcex9xe2x80x2 can vary along the length of the grating.
Long-period gratings are particularly useful in optical communication systems for equalizing amplifier gain at different wavelengths. See, for example, U.S. Pat. No. 5,430,817 issued to A. M. Vengsarkar on Jul. 4, 1995.
Many potential applications require optical gratings having characteristics which are tunable. Tunable Bragg gratings can permit dynamic control of which wavelength will pass through the grating and which will be reflected or diverted. A tunable chirped Bragg grating can permit dynamic dispersion compensation. And a tunable long period grating can provide dynamic gain compensation.
Thermally tunable grating devices control the characteristics of the gratings by the application of heat to change the index of refraction and thus the optical pathlength between successive perturbations. An emerging class of tunable fiber devices uses conventional intracore fiber gratings and thin film resistive heaters with uniform, tapered or periodically varying thicknesses formed in single or multilayer geometries on the fiber surface. Current passing through these films causes distributed Joule heating at rates that depend on the electrical current and the local resistance of the coating. The temperature changes induced by this heating alter the properties of gratings in the core of the fiber. Uniform heating of a fiber Bragg grating, for example, causes a shift in the resonance wavelength proportional to the change in temperature. The shift is due primarily to the intrinsic dependence of the glass index of refraction on temperature. Fiber elongation caused by the thermal expansion also induces smaller shifts.
By using microfabricated distributed heaters with resistances that vary along the length of the fiber grating (typically 2-10 cm long), it is possible to induce and dynamically adjust simple or complex spatial variations in the periodicity of the grating (i.e. chirps). Integrated heaters with thicknesses that depend inversely on position along the fiber have resistances that vary linearly with position. Heating produced by these films provides, to a good approximation, a tunable linear chirp. In reflection mode, Bragg gratings with this design can be used for dynamic per-channel dispersion compensation in high bit-rate lightwave communication systems. These devices are compact, power efficient, cost-effective and simple to build.
In many existing and future applications, it will be necessary to establish and control independently linear and higher order chirps, and to adjust the center position of the reflection band. This functionality can be realized with multiple independent heaters that produce temperature gradients with different functional forms. These heaters are designed with geometries that provide a xe2x80x9cbasis setxe2x80x9d for defining and tuning the desired range of temperatures profiles. We previously demonstrated this approach in a simple system that incorporated one uniform and one tapered heater deposited in a multilayer geometry on the fiber surface. The metal heaters in this case were electrically insulated from one another by a thin film of SiO2 deposited by plasma enhanced chemical vapor deposition. Coordinated control of the heaters allowed, for example, the chirp rate to be adjusted without shifting the center position.
While this multilayer geometry achieves the necessary functionality, it requires the deposition of multiple robust, thin layers on a highly curved object. It can be challenging to reliably produce these coatings from materials that are compatible with the relatively high operating temperatures (e.g. 300xc2x0 C. of these devices) without expensive deposition tools. Also, these designs require multiple processing steps.
Accordingly, there is a need for alternative heater designs that avoid multiple depositions on highly curved fibers but, at the same time, retain all of the advantages of the multilayer thin film devices.
A thermally tunable optical fiber device comprises a length of optical fiber including a device disposed within a microcapillary heater. The microcapillary heater can include a thin film resistive heater. The fiber itself can optionally include a thin film resistive heater overlying the device, and a plurality of nested microcapillary tubes can optionally provide a plurality of successive concentric heaters overlying the device. The heaters films can be films with uniform, tapered or periodically varying thickness. The heaters can be single layer or multiple layer. Multiple layer films can be superimposed with intervening insulating layers or plural layers can be formed on different angular regions of the microcapillary. Thus one can provide virtually any desired temperature versus length profile along the fiber device.